Thermal spraying involves the heat softening of a heat fusible material such as metal, carbide or ceramic, and propelling the softened material in particulate form against a surface which is to be coated. The heated particles strike the surface where they are quenched and bonded thereto. A conventional thermal spray gun is used for the purpose of both heating and propelling the particles. In one type of thermal spray gun, the heat fusible material is supplied to the gun in powder form. Such powders are typically comprised of small particles, e.g., between 100 mesh U.S. Standard screen size (150 microns) and about 5 microns.
The term "flame spraying" as used herein specifically means a combustion spray process as a species of the broader group of thermal spray processes. A thermal spray gun normally utilizes a combustion or plasma flame to produce the heat for melting of the powder particles. It is recognized by those of skill in the art, however, that other heating means may be used as well, such as electric arcs, resistance heaters or induction heaters, and these may be used alone or in combination with other forms of heaters. In a powder-type combustion flame spray gun, the carrier gas, which entrains and transports the powder, can be one of the combustion gases or an inert gas such as nitrogen, or it can be simply compressed air. In a plasma spray gun, the primary plasma gas is generally nitrogen or argon, and hydrogen or helium is usually added to the primary gas.
The material alternatively may be fed into a heating zone in the form of a rod or wire. In the wire type thermal spray gun, the rod or wire of the material to be sprayed is fed into the heating zone formed by a flame of some type, such as a combustion flame, where it is melted or at least heat-softened and atomized, usually by blast gas, and thence propelled in finely divided form onto the surface to be coated. The rod or wire may be conventionally formed as by drawing, or may be formed by sintering together a powder, or by bonding together the powder by means of an organic binder or other suitable binder which disintegrates in the heat of the heating zone, thereby releasing the powder to be sprayed in finely divided form.
Since wear resistance is a common requirement for thermal sprayed coatings, carbide powders have been of considerable interest for spraying. Carbides such as tungsten carbide, without any binder ("neat"), oxidize and lose carbon during the high temperature spraying process. An effort to minimize these effects is disclosed in U.S. Pat. No. 3,419,415, originally assigned to a predecessor in interest of the assignee of the present application, whereby a composite powder is formed of the carbide with excess carbon. However this method has not been particularly successful and apparently has never been commercially developed.
British patent specification No. 867,455, also originally assigned to predecessor in interest of the present assignee, typifies metal bonded carbide powder admixed with a sprayweld self-fluxing alloy powder for spraying. Often the coating is subsequently fused. The addition of fuseable self-fluxing alloy not only adds time and cost to the process but results in a lesser amount of carbide in the coating. U.S. Pat. No. 4,136,230 (Patel) illustrates typical grain sizes of tungsten carbide particles in a self-fluxing alloy matrix in a fused flame sprayed coating.
U.S. Pat. No. 3,023,490 teaches a coating comprising large and small particles of tungsten carbide in a fusible alloy matrix. This coating is formed by applying powders in a paste onto a substrate and torch fusing the coating in place, a process not widely competitive with thermal spraying.
Therefore, tungsten carbide powder developed for thermal spraying has generally required binders of additional materials in the powder. Firstly, since tungsten carbide itself does not melt properly in the flame, and also is too brittle for practical coatings, a metal such as cobalt or nickel is incorporated into the powder. Such a powder is produced by fusing or sintering with the metal, and crushing the product, as taught in the aforementioned British patent. Secondly, combustion flame spraying tends to oxidize and decarburize neat metal bonded carbide powder. Also thermal spraying tends to cause the carbide to go into solution in the matrix. High velocity plasma minimizes these effects to produce excellent results. However for combustion flames spray processes the powder is generally admixed with another flame spray material.
When plasma and detonation processes were developed around 1960, the spraying of powders such as cobalt bonded tungsten carbide (without admixture) became quite successful for producing highly wear resistant coatings. However the apparatus for these processes are expensive and not very portable, thus limiting applications. The more portable and economically reasonable combustion flame spray processes still have generally not been successful in spraying high quality cobalt bonded tungsten carbide coatings without added self-fluxing alloy.